In the packaging industry there is an increasing demand for thermoplastic films exhibiting especially excellent mechanical properties, a low friction coefficient, a pleasant feel, and an excellent sealing behaviour.
With regard to the sealing of thermoplastic films, it is recognized that in the case of films of thickness greater than 100 microns, the behaviour of the seals produced by a high-frequency current is superior to the behaviour of the seals produced thermally. However, the sealing times using high-frequency current are generally high (2 to 3 seconds, even longer), and this is nowadays unacceptable in the modern packaging industry, where packaging rates are increasingly fast. In the sector now being considered it is agreed nowadays that a sealing time is short when it does not exceed one second.
International Patent Application WO 92/16358 describes a sealing process using high-frequency current--which will be referred to as HF sealing in what follows--of at least two thermoplastic films which include a copolymer or a copolymer-polymer mixture chosen from the group consisting of an ethylene/vinyl acetate (EVA) copolymer, a mixture of EVA and of a polyethylene, a mixture of EVA and of a polypropylene, or a mixture of EVA, polyethylene, and polypropylene. In these mixtures, or else in the case of the use of an EVA by itself, the vinyl acetate weight content is between 12% and 28%.
The process described in this international application consists in: pressing at least two thermoplastic layers, applying an HF current to the said pressed layers with a view to sealing them together, and recovering the said sealed layers.
Although this procedure gives full satisfaction insofar as the quality of the seal and the mechanical properties of the sealed films are concerned, no information is given in this international application concerning sealing rates and products which would be capable of being obtained at such rates.
Numerous investigations into the rates of HF sealing together of at least two thermoplastic films based on ethylene copolymer indicate that to obtain sealing times lower than or at most equal to one second, it was necessary to increase the energy per unit area of the electrode permitting the HF sealing. It was found, however, that, in the case of high energies, the HF sealing times were actually very short, but that flashes were produced which resulted in the deterioration of the films (tears, holes, high colouring). These flashes, consequently, restricted the energies employed.
It was also found that high comonomer contents in the ethylene copolymers allowed very short HF sealing times. Unfortunately, products with high comonomer contents are difficult to extrude because they are much too sticky, and this is unacceptable on modern packaging machines.